How to Jump-Start a Car Safely

A flat battery is an inconvenience, not an emergency, and a jump-start gets you moving again in a few minutes. The reason it deserves a careful guide is that the two things being connected are a large amount of stored electrical energy and a battery giving off hydrogen gas. Get the order right and it is routine. Get it wrong and you risk a spark in the worst possible place, or damage to expensive electronics in both cars.

Read your manual first

This is not a formality. On modern cars it genuinely changes what you do.

• Many cars no longer have accessible battery terminals under the bonnet. Instead the manufacturer provides a remote positive post and a designated earth point, often under a cover marked with a plus symbol.
• Some manufacturers advise against jump-starting their vehicle, or against using it as the donor car, because of the load it places on the electrical system.
• Hybrid and electric cars have a 12 V battery as well as the high-voltage traction battery, and the procedure is specific. Never connect anything to a high-voltage system. Many hybrids must not be used as a donor vehicle at all.
• Some cars require the ignition in a particular position, or the bonnet catch released in a particular way.

Two minutes with the manual saves a great deal of trouble.

When not to jump-start at all

Call for assistance instead if:

• The battery case is cracked, leaking, bulging or smells strongly of rotten eggs
• The battery is frozen
• You can see or smell fuel around the car
• The car has been in a collision
• You are not confident about which terminal is which

What you need

• A set of jump leads heavy enough for the job. Thin, cheap leads heat up and do not carry enough current to start a larger engine.
• A donor car with a healthy battery of the same nominal voltage, which for ordinary cars means 12 V.
• Eye protection if you have it.

A portable jump pack is an alternative and avoids needing a second car. If you use one, follow the instructions that came with it — the connection sequence is usually simpler because the pack is designed for the job.

Before you connect anything

1. Position the donor car close enough for the leads to reach comfortably, but do not let the two cars touch. Contact between the bodies creates an unintended circuit.
2. Switch both cars off completely. Remove the keys.
3. Apply the parking brake on both. Automatics in Park, manuals in neutral.
4. Switch off headlights, air conditioning, stereo and anything else drawing power in both cars.
5. Open both bonnets and identify the positive and negative connection points. Positive is usually red and marked with a plus; negative is usually black and marked with a minus.
6. Remove rings, watches and dangling jewellery, and keep loose clothing and hair away from belts and the cooling fan, which can start without warning.

Connecting, in order

The order matters. Follow it exactly.

1. Red clamp to the flat car’s positive terminal or designated positive jump post.
2. Red clamp at the other end to the donor car’s positive terminal.
3. Black clamp to the donor car’s negative terminal or its designated earth point.
4. Black clamp at the other end to a bare, unpainted metal earth point on the flat car — an engine lifting eye or a solid bracket on the block — well away from the battery, the fuel lines and any moving parts. Your manual names the point to use.

That last step is the one people get wrong. Connecting the final clamp directly to the flat battery’s negative terminal makes the inevitable small spark happen right next to a battery venting hydrogen. Earthing to the engine or chassis puts that spark somewhere harmless.

Check that no lead is resting against a belt, a pulley or the cooling fan.

Starting

1. Start the donor car and let it run for two or three minutes at a fast idle. This puts some charge into the flat battery rather than asking it to do everything at once.
2. Try to start the flat car.
3. If it does not fire within about ten seconds, stop. Wait a minute or two before trying again — continuous cranking overheats the starter motor.
4. If it will not start after three or four attempts, the problem is probably not simply a flat battery. Stop and get it looked at.

Disconnecting, in reverse order

With both engines running:

1. Black clamp off the earth point on the recovered car
2. Black clamp off the donor car
3. Red clamp off the donor car
4. Red clamp off the recovered car

Keep the clamps from touching each other or any metal while the other end is still attached.

Afterwards

The battery is nowhere near full. Drive for a decent run, or use a proper battery charger at home, which is better for the battery than a short drive.

Then think about why it went flat:

• A light or a door left ajar overnight explains one flat battery
• A battery over three years old that has flattened once will usually do it again
• If the charging warning light is on, or the car goes flat again after a good run, the charging system or the battery needs testing rather than another jump

A jump-start is a way to get home, not a repair.

Your manual has the final word

Jump points, earth points, whether jump-starting is approved at all, and the procedure for hybrids and electric cars are all specific to your vehicle. Where this guide and your owner’s manual disagree, follow the manual without exception.